Service Worker registration error: Unsupported MIME type ('text/html')

Question

I'm using create-react-app with an express server.

create-react-app has a pre-configured ServiceWorker that caches local assets (https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#making-a-progressive-web-app).

The problem I encountered when I tried to publish on my server is that the service-worker.js file was available, but I was getting errors in my browser console when it tried to register it.

On Firefox, I got this error:

Error during service worker registration:

TypeError: ServiceWorker script at https://my-domain.com/service-worker.js for scope https://my-domain.com/ encountered an error during installation.

On Chrome, I get the more descriptive error:

Error during service worker registration: DOMException: Failed to register a ServiceWorker: The script has an unsupported MIME type ('text/html').

Sure enough, if I look in the Network tab of my developer tools and search for the service-worker.js file, I can see that it has the wrong MIME type in the HTTP headers.

I could not understand, though, why it has the wrong MIME type?

Answer 1

In my Express server application, I have one wildcard (asterisk / *) route that redirects to index.html:

// Load React App
// Serve HTML file for production
if (env.name === "production") {
  app.get("*", function response(req, res) {
    res.sendFile(path.join(__dirname, "public", "index.html"));
  });
}

This is a very common design pattern. However, it means that any requests for unknown text files initially get redirected to index.html, and therefore return with the MIME type of "text/html", even if it's actually a JavaScript or SVG or some other kind of plaintext file.

The solution I found was to add a specific route for service-worker.js before the wildcard route:

app.get("/service-worker.js", (req, res) => {
  res.sendFile(path.resolve(__dirname, "public", "service-worker.js"));
});
app.get("*", function response(req, res) {
  res.sendFile(path.join(__dirname, "public", "index.html"));
});

Now, when the browser looks for service-worker.js, Express will return it with the correct MIME type.

(Note that if you try adding the service-worker.js route after the wildcard, it won't work because the wildcard route will override.)